Wiardgroup

Hi Justin
     i hope you will forgive the intrusion, as your email
specifically addressed Grant. And  i hope Grant will respond as well
because you can bet he will have something interesting to say. But
your question is just too juicy to pass up. It touches on a lot of
very critical issues concerning analog module designs.
     An "OTA" (Operational Transconductance Amplifier) can be
used as a PRECISION voltage controlled resistor (it has other
applications as well but that's a whole `nother story).
Analog synthesis is one of the realms where voltage control is STILL
, pretty much, THE crucial method of control.  A device that turns a
voltage into a resistance is one of the handiest things designers can
have in their toolkits when they set out to steer voltage on complex
courses.  Now, there are a bunch of components that can pull this
conversion off. Diodes, Transistors,  Photoresistors, even
M.O.V's can do this trick too. But the distinguishing feature of
the OTA is it's PRECISION. i don't know of any other analog
component that yields 1v/octave scaling as painlessly as an OTA.
That's why they're so desirable. Tom Henry of Midwest Analog 
Products wrote a terrific little book called `Making Music with
the 3080 OTA.' 
http://www.midwest-analog.com/catbooks.html
It's a great source of information on  OTA's in general, not
just the venerable and capricious 3080.
      Another issue is why they're becoming so rare.  The quick
and dirty answer is that `science' marches on, but money
SPRINTS ahead. It's a digital world now. The entire commercial
analog hardware `industry' is barely more than a hobby for
about a couple dozen people. It's ENTIRE annual financial
input/output is the   equivalent of  one smallish owner-operated
american tavern.   Digital hardware designs and software yield much
greater precision and economy. The synthesizer - industry – moved
there 20 years ago, so thoroughly in fact, that even experts are now
hard-pressed to discern quickly whether an order for that synth you
see in an EM ad will bring an actual  synthesizer, or a little
cardboard box with a disc in it to your house (hint: the price is the
best criterion). Digital hardware (and so much less, softsynths) have
no need for OTA's.  The cheapest DAC can outperform the best OTA
in a contest for precision. So just like steam engines the economy
sets the context and OTA's are obsolete. 
     So everybody knows that they don't make ARP2600's
anymore but it's less widely known that they don't make the
components with which you could MAKE (or repair) ARP2600's
anymore either. Modern hardware design is standardized on
surface-mount (SOIC) chips (even Grant  used this method for the
noiz-ring). These chips are TINY – about the size of a booger .
They have the advantage of being cheap,  and plentiful and, to be
fair, many of the `greatest hits' of the analog chip world
are still being manufactured in SOIC packages. But these lilliputian
critters are designed for the convenience of automated assembly lines
not human fingers. As Grant has pointed-out, manufacturers have
stopped making many OTA types altogether  So the only stocks in
existence are in the discontinued stock of distributors and prices 
are adjusted accordingly until they're all sold and then it
becomes an auction. In this sense EVERY piece of analog gear is a
limited edition!
      This screed is not the whole story of course, there are many
corollaries to the axiom "all things must pass' (the tube
renaissance, handmade discrete op-amps etc.) but beware! I can
prattle on for hours about that too! 
      i can't say this enough:  get `em while you can.